Air sack on chickens?? Please Help!

We have had chickens for a long time, and about a month ago one hen had an air bubble on her right shoulder. A few weeks later, she died. Last week i noticed that on one of my roosters he had the same air sack on the same shoulder. What is it? And how Can it be treated??? Also, yesterday, i saw on one of my baby chickens she had it too! I dont want them to die so can anyone give my some advice? Do your chickens have air sacks??

This isn't my pic, but it is very similar. This is what the air sack looks like. Also, it has veins and is about the size of a penny. I couldnt get a pic up, so here is one that looks exactly like it. Not the food sack.